1. Field of the Invention
The present invention relates to a power amplifier of a radio transmitter. More particularly, the present invention relates to a technology of calibrating the characteristic of a linear amplifying stage of a hybrid supply modulator that modulates a supply voltage of a power amplifier of a radio transmitter.
2. Description of the Related Art
For the sake of long battery use time for a wireless mobile communication terminal a power management integrated circuit and a method for efficiency enhancement of a wireless power amplifier is desired. In a Wireless Broadband (WiBro) system and a Long Term Evolution (LTE) system, a wireless mobile communication terminal uses a technology for obtaining a characteristic of high efficiency despite a characteristic of high Peak-to-Average Power Ratio (PAPR). An exemplary technology corresponding to this is an Envelope Tracking (ET) or Envelope Elimination and Restoration (EER) technology. The ET or EER technology varies a supply voltage of a Radio Frequency (RF) power amplifier in accordance with the output power of the RF power amplifier and operates the RF power amplifier in a saturated region or switching region to have both a high linearity and a high efficiency characteristic. Particularly, despite a modulated signal having a high PAPR, an RF linear amplifier can have a high efficiency.
FIG. 1 illustrates a power amplifier architecture according to the related art.
Referring to FIG. 1, a modulator/demodulator (modem) 100 processes a baseband signal in accordance with a corresponding communication scheme (e.g., an Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) communication scheme or a Code Division Multiple Access (CDMA) communication scheme) and outputs the baseband signal to a Radio Frequency (RF) module 102. Also, the modem 100 provides an envelope component of the baseband signal to a supply modulator 106. The RF module 102 converts the received baseband signal into an RF signal, and outputs the RF signal to an RF power amplifier 104.
The supply modulator 106 modulates a Direct Current (DC) source (e.g., battery power) in accordance with an envelope signal provided from the modem 100, and outputs an Alternating Current (AC) source to the RF power amplifier 104. An output signal of the supply modulator 106 is used as a voltage source, and has optimal linearity and efficiency.
The RF power amplifier 104 amplifies an RF signal depending on an output signal of the supply modulator 106, and outputs the amplified RF signal through an antenna. By using the ET technique or EER technique, the RF power amplifier 104 can amplify the RF signal depending on the output signal of the supply modulator 106.
FIG. 2 illustrates a power amplifier architecture using a hybrid supply modulator according to the related art.
Referring to FIG. 2, a modem 200, an RF module 202, and an RF power amplifier 205 are the same as the modem 100, the RF module 102, and the RF power amplifier 104 of FIG. 1 and thus, a detailed description thereof is omitted herein.
The RF power amplifier 205 of FIG. 2 uses the hybrid supply modulator, which is composed of a linear amplifying stage and a switching amplifying stage. The majority of the current supplied to the RF power amplifier 205 is supplied from the switching amplifying stage of the hybrid supply modulator. The linear amplifying stage pushes and pulls compensating currents so as to compensate for a linear distortion, which is caused by a ripple characteristic included in an output signal (hereinafter, referred to as ‘switching currents’) of the switching amplifying stage when the switching currents pass through an inductor 212. In other words, when the output signal of the switching amplifying stage is low, the linear amplifying stage pushes current to the output signal of the switching amplifying stage and, when the output signal of the switching amplifying stage is high, the linear amplifying stage pulls current from the output signal of the switching amplifying stage. As the architecture of the switching amplifying stage of the hybrid supply modulator, it generally uses a buck converter.
The linear amplifying stage maintains a desired gain characteristic of an output voltage of the hybrid supply modulator versus an input envelope signal thereof, and has a feedback loop for amplification and power conversion. By means of a feedback signal, the linear amplifying stage performs comparison between an output signal of the linear amplifying stage and the input envelope signal thereof and compensates for the non-linearity of the switching amplifying stage.
Generally, the linear amplifying stage includes feedback resistors 208 and 210 and a linear amplifier 214.
The switching amplifying stage includes a comparator 216 for generating a switching signal and a switching amplifier 218 for amplifying the switching signal.
The linear amplifier 214 includes an Operational Trans-conductance Amplifier (OTA) input stage (or OA amp) (not shown) having a high gain, a class-AB bias stage (not shown), a push-pull output stage (not shown), and a stability compensation feedback Resistor (R) and Capacitor (C). On the other hand, the linear amplifier 214 includes an output current sensing stage for sensing currents of the push-pull output stage and generating a switching signal for driving the switching amplifying stage.
The OTA input stage having a high gain characteristic generates a control signal for controlling to make an input reference signal and an output voltage identical with each other. The class-AB bias stage enhances an efficiency characteristic of the push-pull output stage and supplies a bias to have a desired linear characteristic. The push-pull output stage outputs a current component of a high frequency domain that the switching amplifying stage fails to provide sufficiently. Accordingly, a transistor level of the push-pull output stage is determined based on a maximum value upon design. However, the maximum level of the push-pull output stage causes a big parasitic capacitance, deteriorating a BandWidth (BW) characteristic of the linear amplifying stage. Also, the linear amplifier 214 requires a Resistor-Capacitor (RC) compensation circuit to provide a sufficient phase margin.
Accordingly, the linear amplifying stage should have linear characteristics for a signal having a wide bandwidth and therefore, should have a high Direct Current (DC) gain and a high Gain-Bandwidth product (GBW). Also, the linear amplifying stage is able to supply a large current to the RF power amplifier 205. The linear amplifying stage has a low output resistance value in order to compensate for ripple currents generated from the switching amplifying stage. The linear amplifying stage performs a rail-to-rail operation for the sake of a high output voltage.
Meantime, an output load resistance of the hybrid supply modulator for an ET amplifier is equivalent to a resistance 204 presented in a drain or collector of the RF power amplifier 205. The output load resistance of the hybrid supply modulator is varied in size depending on a power level of an input signal of the hybrid supply modulator. As the output load resistance value is varied, it is difficult to sufficiently maintain a phase margin characteristic of the linear amplifying stage that is designed for a specific load resistance value.
FIG. 3 is a graph illustrating a variation of the second pole dependent on a variation of an output load resistance value according to the related art.
Referring to FIG. 3, as an output resistance of the hybrid supply modulator is varied, the second pole (ωp2) of the linear amplifying stage is varied. As the second pole (ωp2) is varied, even a phase margin characteristic of the linear amplifying stage is varied. That is, a high load resistance value causes a shift of the second pole (ωp2) to a low frequency domain, causing a deterioration of the phase margin characteristic of the linear amplifying stage. Therefore, at low RF input signal operation, the hybrid supply modulator is placed in an oscillation or unstable state.
Accordingly, a parasitic capacitance of a high-level transistor used in the push-pull output stage used for supplying large currents to the RF power amplifier 205 results in the shift of the second pole (ωp2) to the low frequency domain, causing the failure of the linear amplifying stage to have a sufficiently high GBW. Therefore, because a ripple existing at a high frequency is not compensated by the linear amplifying stage, it deteriorates the spectrum characteristic of an output stage of the hybrid supply modulator.
Also, in order the push-pull output stage to maintain a sufficient linear characteristic and obtain a high efficiency characteristic, the push-pull output stage requires a class-AB bias. However, the class-AB bias is vulnerable to a characteristic of Process Voltage Temperature (PVT) variation that is the characteristic of the RF power amplifier. That is, a class-AB bias point is varied into a class-B bias point or class-C bias point in accordance with the PVT variation.
FIG. 4 illustrates a non-linear operation characteristic dependent on a bias variation of a push-pull output stage in a linear amplifying stage according to the related art.
Referring to FIG. 4, if a push-pull output stage 400 pushes and pulls current properly at a corresponding bias point, an output signal of a hybrid supply modulator has a good linear characteristic in step 410. If the push-pull output stage 400 fails to push and pull current properly at a corresponding bias point, an output signal of the hybrid supply modulator cannot maintain a linear characteristic, and has a distortion component, i.e., has a bad linear characteristic in step 420.
The instable characteristic or non-linear characteristic of the hybrid supply modulator generates a noise component or spurious component in an output of an envelope tracking amplifier, deteriorating an RF spectrum characteristic, and a noise characteristic at a receive stage band.
Accordingly, there is a need for an apparatus and method for power amplification for calibration capable of improving the stability, frequency characteristic, and linearity of a hybrid supply modulator in a transmitter.